A magnetic element adapted to perform a thermally-assisted switching (TAS) read and write operation and comprising a magnetic tunnel junction is described in U.S. Pat. No. 6,950,335. As shown in FIG. 1, the magnetic element 1 comprises a magnetic tunnel junction 2 formed from a tunnel barrier 22 being disposed between a first magnetic layer, or reference layer 21, and a second magnetic layer, or storage layer 23. The magnetic element 1 further comprises an upper current line 4 connected at the upper end of the magnetic tunnel junction 2, and a strap portion 7 extending laterally and substantially parallel to the first and second magnetic layers 21, 23 and connecting the bottom end of the magnetic tunnel junction 2 to a select transistor 3. The configuration strap portion 7 is advantageous for disposing a bottom current line 5 which uses will be discussed below.
The reference layer 21 can be formed from a Fe, Co or Ni based alloy and have a first magnetization that having a fixed magnetization direction. The first magnetization can be fixed in any conventional manner, such as by using a high coercivity (or large switching magnetic field) material. For example, the direction of the first magnetization can be fixed by being exchange-coupled to an antiferromagnetic reference layer (not shown) pinning the first magnetization at a low threshold temperature, below the reference blocking temperature TBR of the antiferromagnetic reference layer.
Preferably, the tunnel barrier 22 is a thin layer, typically in the nanometer range and is formed, for example, from any suitable insulating material, such as alumina or magnesium oxide. The tunnel barrier 22 has typically a resistance-area product smaller than 50 Ω·m2.
The storage layer 23 can have a second magnetization which direction can be freely adjusted when the magnetic tunnel junction 2 is heated at a high threshold temperature. The storage layer 23 can be a layer of ferromagnetic material typically including Fe, Co, Ni or their alloys. The storage layer 23 can be exchange-coupled with an adjacent antiferromagnetic storage layer (not shown) pinning the storage layer 23 at a temperature below a storage blocking temperature TBS of the antiferromagnetic storage layer, where TBS is preferably smaller than the storage blocking temperature TBR. The antiferromagnetic storage layer can be made of an alloy of Fe and Mn, such as FeMn, or Ir and Mn, for example, an alloy containing 20% of Ir and 80% of Mn. The storage blocking temperature TBS can be typically comprised between 150° C. and 250°. During the TAS write operation, the magnetic tunnel junction 2 is heated at the high threshold temperature, above TBS but below TBR, by applying a heating current 31 to the magnetic tunnel junction 2. The heating current 31 can be applied via the current line 4, when the select transistor 3 is in the passing mode. Once the magnetic tunnel junction 2 is heated at the high threshold temperature, the direction of the second magnetization can be adjusted by by passing a spin polarized electric current or a current induced magnetic switching (CIMS) in the magnetic tunnel junction 2, or by using an external magnetic field 52 as shown in FIG. 1. In the latter case, the external magnetic field 52 can be generated by a passing a field current 51 in the bottom current line 5. The position of the bottom current line 5 in the vicinity of the storage layer 23 allows for minimizing the field current 51 used. During writing, the second magnetization can be oriented such as to be substantially parallel or antiparallel (as in the example of FIG. 1) with the first magnetization direction. The second magnetization is then frozen in its switched direction by inhibiting the heating current 31, cooling the magnetic tunnel junction 2 at a low threshold temperature, below TBS.
During the read operation, the resistance of the magnetic tunnel junction 2 can be measured by passing a sense current (not shown) through it. The measured resistance varies depending on the relative directions of the first and second magnetizations. A high resistance is measured when the first magnetization is oriented substantially antiparallel with the second magnetization and a small resistance is measure when the first and second magnetizations are oriented substantially parallel.
Passing the heating current 31 for heating the magnetic tunnel junction 20 to the predetermined high temperature threshold, requires applying a voltage Vheat, having a possibly high value, between the reference and storage layers 1, 4.
In FIG. 2, the potential energy E is plotted against a distance X along the magnetic tunnel junction 2, representing the potential energy function 8 of the electrons in the magnetic tunnel junction 2 subjected to a potential Vheat, generated when the heating current 31 is passed in the magnetic tunnel junction 2. In FIG. 2, the reference and storage layers 21, 23 are located on each side of the tunnel barrier 22, placed at X0. For negative values of the heating current 31, the reference and storage layers 21, 23 behave like an electron-emitting layer having an upper Fermi level Efs, and an electron-receiving layer having a lower Fermi level Efi, respectively. The difference of the Fermi levels is proportional to the potential difference: Efs−Efi=eV, e being the elementary charge of the electron. As illustrated by the arrow 9, an electron emitted by the emitting layer passes through the tunnel barrier 22, by tunnel effect, without dissipating energy. When inelastic relaxation of the electron from a higher energy Efs to a lower energy Efi takes place, the electron dissipates the energy eV in the electron-receiving layer, for example by creation of phonons 10 and/or magnons 11, which increases the temperature of the electron-receiving layer. Inelastic relaxation takes place over a characteristic length, or inelastic mean free path, λin, which length is typically about a few nanometers in the magnetic materials usually used in typical magnetic tunnel junctions. Heat production by the electrons passing through the tunnel barrier 22, or tunnel current (not shown), is thus maximal in a zone with a thickness of a few nanometers, located in the receiving layer and adjacent to the tunnel barrier 22. Since the potential Vheat used for heating the magnetic tunnel junction 2 at the high temperature threshold can be high, such heat production can damage the magnetic tunnel junction 2.
It can be advantageous to thermally insulate the magnetic tunnel junction 2 such that during the TAS write operation, the magnetic tunnel junction 2 is heated efficiently with a reduced heating current 31, thus minimizing the electric power necessary for performing the write operation.
FIG. 3 illustrates the magnetic element 1 according to European Patent Application No. 1,671,330, where the magnetic tunnel junction 2 further comprises an upper thermal barrier 14 placed between the current line 4 and the reference layer 21, and a bottom thermal barrier 15, between the storage layer 23 and the strap portion 7. In FIG. 3, a bottom electrode 16 is represented, connected to the strap portion 7 through a bottom via 17. The select transistor 3, not represented in FIG. 3, can be connected to the bottom electrode 16 or directly to the bottom via 17. During the TAS write operation, the upper and bottom thermal barrier 14, 15 advantageously reduce heat losses at both upper and bottom ends of the magnetic tunnel junction 2, when the heating current 31 is passed. Consequently, a more efficient heating of the magnetic tunnel junction 20 is achieved and the heating current 31, or potential Vheat, needed for heating the magnetic tunnel junction 20 can be lowered.
Since the thermal barriers 14, 15 are connected in series with the tunnel barrier 22, their electrical conductivity must be high enough compared to that of the tunnel barrier 22 to ensure that the electrical current flows uniquely through the magnetic tunnel junction 2. Preferably, the electrical conductivity of the thermal barriers 14, 15 is higher by a factor ten compared with that of the tunnel barrier 22. This limits the possibility of using material with very low thermal conductivity for the thermal barriers 14, 15. Typically, such thermal barriers are made from an alloy containing Bismuth (Bi) and Tellurium (Te), such as BiTe, which exhibits an electric conductivity of about 1.75 mΩ-cm and a thermal conductivity of about 1.5 W m−1° C.−1.